Studies with DNAs isolated from 19 MCF247 induced tumors have continued with emphasis on a more complete evaluation of one specific tumor. The 19 tumors divide into three classes with respect to the expression of the Ly-2 surface antigen: 9 of the tumors are Ly-2 +; 3 are Ly-2 -; and the remaining 7 tumors contain a 50-50% mixture of Ly-2 + and Ly-2 - (Ly-2 +/-). Cells from one of the Ly-2 +/- tumors (Ty-3) were inoculated into 5 newborn AKR/J mice. Evaluation of the tumors that developed in these mice showed that one tumor was Ly-2 -, and that the other 4 were again Ly-2 +/-. DNAs isolated from these 5 passage tumors were evaluated for the presence of restriction fragments that hybridize to MuLV ecotropic env and xenotropic env DNA probes. No difference in patterns were observed when compared to DNA from the primary tumor, suggesting that in vivo passage of the established tumor cells is not associated with an alteration in the integrated MuLVs present in the tumor cell genome. Thus a source of material for further evaluation of the role of MuLVs in the development of murine tumors has been established. Further cloning of novel MuLV insertion sequences in these tumors is in progress. To aid in construction of restriction maps of cloned and genomic DNA, a program was written for the IBM-PC which reduces the effort required to determine the sizes of fragments produced after digestion with restriction enzymes. An improved fitting algorithm, utilizing a cubic spline instead of the more commonly used third order polynominal, results in a more accurate estimation of fragment size.